Currently, defects in wafers can be detected by comparing a target die fabricated on the wafer to reference dies fabricated on the wafer. Inspection systems accomplish this by taking images of the target and reference dies for comparison purposes. In particular, detecting the defects often involves performing two separate comparisons to generate two separate results, one comparison being between the target die and one of the reference dies and another comparison being between the target die and the other one of the reference dies. Any similarity between the two separate comparison results is generally used as an indicator of a defect in the target die. This process is known as double difference detection, which can be applied to process window qualification (PWQ) wafers, focus exposure matrix (FEM) wafers, non-modulated wafers, etc.
Prior art FIG. 1 shows traditional layout for a PWQ wafer having a plurality of target dies in a column 102, each being a same pattern modulated (i.e. amplified) by a different combination of parameter (e.g. focus (F) and exposure (E)) values, and further having a plurality of reference dies in columns 104, 106 situated on either side of the column of target dies and each being a nominal (i.e. not modulated) version of the same pattern. Thus, for any particular one of the target dies in column 102, a reference die from column 104 and a reference die from column 106 may be used for detecting defects in the particular target die (see box 108) using double difference detection. While the reference dies are shown as being adjacent to the target die, this is not necessarily always the case. For example, in other wafer configurations the reference dies for any particular target die may be those closest, but not necessarily adjacent, to the particular target component.
In another well-known embodiment (not shown), FEM wafers have a matrix of dies where the parameters of the dies are modulated in a matrix layout. In this embodiment, a center-most die may be nominal, or at least the most nominal of all of the dies on the wafer. Double difference detection may be applied to any target die on the wafer with corresponding reference dies being those adjacent to the target die within the matrix.
Unfortunately, the accuracy of results obtained by these die-to-die comparisons can be negatively affected due to non-critical differences in the target and reference dies. For example, purposeful modulation of target and/or reference dies in process window qualification (PWQ) wafers and focus exposure matrix (FEM) wafers inherently results in these non-critical differences which may be falsely detected as defects during double difference detection. Of course, non-modulated wafers can similarly be affected due to inadvertent but non-critical differences across die images due to error within the inspection system itself.
Prior art FIG. 2 shows an example of the effect of traditional defect detection methods where modulation of only the target component is employed. In FIG. 2, as the modulation is increased for the target component, the size of each part of the target component (including defects and non-defects) also increases, thus causing differentiation between the target and reference components on a part-by-part basis regardless of actual defect. As shown, at higher modulation the difference image resulting from the comparisons includes additional differences than at lower modulations. Existing patents disclosing the above described prior art techniques include U.S. Pat. No. 8,213,704 and U.S. Pat. No. 6,902,855, the descriptions of which are incorporated by the reference in their entirety.
There is thus a need for addressing these and/or other issues associated with the prior art techniques used for defect detection in fabricated components.